JEOS:RP - Journal of the European Optical Society Rapid publications
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Modelling adapted to manufacturing aspects of holographic grating structures
Get PDFThe diffraction efficiencies of modified sinusoidal and blazed gratings are investigated in the high spatial frequency regime by rigorous numerical methods and are compared to experimentally manufactured gratings. The introduced modifications take actual technological induced variations of the profile geometries, such as specific corner rounding, into account. The high spatial frequency regime (resonance regime) is characterized by a local grating period, g, to wavelength, λ, ratio of 0.7 ≤ g/λ ≤ 4 and shows an important relevance for applications in spectroscopy and diffractive imaging. The investigations are carried out for both reflection on metallic surfaces and transmission of dielectric structures over a broad range of grating periods and incidence angles. It was found that near the grating resonance, the more simply producible sine gratings can compete in diffraction efficiency with sawtooth structures. Additionally, for certain application conditions, holographically modified sine structures achieve higher efficiencies than the ideal sine profile. It is also shown that holographical sinusoidal-like profiles measured by AFM can be fitted to a super-Gaussian shape, which is then used to inversely reconstruct the structure profiles from efficiency data
Investigating the use of terahertz pulsed time domain reflection imaging for the study of fabric layers of an Egyptian mummy
Get PDFThis paper reports the first use of terahertz time domain reflection imaging involving textiles on part of a complete human mummy, still in original wrapping. X-ray technique has been used extensively to investigate anatomical features, since X-ray pass through the wrapping. Terahertz waves, on the other hand, can penetrate into non-metallic materials and its reflection depends on the refractive index of materials at the interface, such as textiles and the air. The mummy of Kharushere (ca. 945-712 B.C.) was examined by using Terahertz time domain reflection imaging in the Egyptian galleries of The Metropolitan Museum of Art. Experimental results suggest that the Terahetz imaging is a promising technique for probing the fabric layers surrounding Egyptian mummies, although it is still very limited in its current state. In the future it could become a useful complement to CT scanning when materials with low radiographic density and contrast are being investigated
Focusing of electromagnetic waves into a dielectric slab. II. Numerical results
Get PDFBased on existing exact and asymptotic Kirchhoff solutions for focused electromagnetic fields inside a dielectric slab we present numerical comparisons between them also for the special cases of focusing through a single interface or in a single medium. These comparisons show that the exact and asymptotic Kirchhoff solutions for focusing in a single medium or through a single interface agree well, except at observation points near to the interface while a small difference between the two solutions for the focused electrical
field inside a dielectric slab is observed, especially at observation points near to one of the interfaces. This difference is believed to be due to contributions from surface waves, which are not accounted for in the asymptotic Kirchhoff solutions. At low Fresnel numbers focal shift phenomena are observed in all three cases
Discontinuous space variant sub-wavelength structures for generating radially polarized light in visible region
Get PDFA discontinuous space variant sub-wavelength dielectric grating is designed and fabricated for generating radially polarized light in visible region (λ = 632.8 nm). The design is based on sub-wavelength silicon nitride structures introducing a retardation of π/2 by form birefringence, with space variant orientation of the optical axis. The pattern is divided into concentric ring segments with constant structural parameters, therefore reducing electron-beam writing time significantly. The design avoids the technological challenges encountered in the generation of a continuous space variant grating while maintaining good quality of the resulting polarization mode
Quantifying the 2.5D imaging performance of digital holographic systems
Get PDFDigital holographic systems are a class of two step, opto-numerical, three-dimensional imaging techniques. The role of the digital camera in limiting the resolution and ï¬eld of view of the reconstructed image, and the interaction of these limits with a general optical system is poorly understood. The linear canonical transform describes any optical system consisting of lenses and/or free space in a uniï¬ed manner. Expressions derived using it are parametrised in terms of the parameters of the optical system, as well as those of the digital camera: aperture size, pixel size and pixel pitch. We develop rules of thumb for selecting an optical system to minimise mean squared error for given input and digital camera parameters. In the limit, our results constitute a point spread function analysis. The results presented in this paper will allow digital holography practitioners to select an optical system to maximise the quality of their reconstructed image using a priori knowledge of the camera and object
The exact theory for scattering of waves by thick holes in a slab and other objects with non-separable geometries
Get PDFThe theory for scattering of electromagnetic waves is developed for scattering
objects for which the natural modes of the field inside the object
do not couple one-to-one with those outside the scatterer. Key
feature of the calculation of the scattered fields is the
introduction of a new set of modes. As an example, we calculate the
reflected and transmitted fields generated by an electromagnetic
plane wave that impinges upon a multilayer slab of which the layers
are stacked perpendicular to the boundary planes.
As this is the geometry of a thick plate with slits our theory encompasses the exact scattering theory of electromagnetic waves by a thick plate with slits
Transition from amplified spontaneous emission to laser action in disordered media of R6G dye and TiO2 nanoparticles doped with PMMA polymer
Get PDFA random laser (RL) based on organic Rhodamine 6G (R6G) laser- dye and TiO2 suspended nanoparticles have been prepared with polymethylmethacrylate (PMMA) as a host. Both liquid and spray-coated homogeneous film samples of 22.4-30.1µm thickness range were use. Optimum concentrations have been determined depending on the normal fluorescence spectra which give evidence that the laser dye provides amplification and TiO2 nanoparticles act as scatter center. At the optimum concentrations, results of the random laser (RL) under second harmonic Nd: YAG laser excitation show that the values of bandwidth at full width half-maximum (FWHM) and the threshold energy are about 9 nm and 15 mJ respectively, which represent the minimum value for the liquid samples in the current research. Correspondly, these values become 14 nm and 15 mJ for film sample. The broadening that can be attributed to the concentration quenching of a laser dye at high a concentration level has been observed
Development and experimental validation of a versatile prototype Swing Arm Profilometer for measuring optical surfaces
Get PDFA versatile prototype Swing Arm Profilometer (SAP) is developed and experimentally validated in the Institute of Optics and Electronics (IOE), Chinese Academy of Sciences (CAS). The configurations for the SAP are introduced. The measurement principles for measuring plano, spherical and aspherical mirrors are given. Four validation experiments are performed on the prototype SAP using plano, spherical and aspherical mirror samples. The measurement uncertainty for the current prototype SAP is 0.5micr
Partially coherent surface plasmon modes
Get PDFElementary long-range plasmon modes are described assuming an exponential dependence of the refractive index in the neighbourhood of the interface dielectric-metal thin film. The study is performed using coupling mode theory. The interference between two long-range plasmon modes generated that way allows the synthesis of surface sinusoidal plasmon modes, which can be considered as completely coherent generalized plasmon modes. These sinusoidal plasmon modes are used for the synthesis of new partially coherent surface plasmon modes, which are obtained by means of an incoherent superposition of sinusoidal plasmon modes where the period of each one is considered as a random variable. The kinds of surface modes generated have an easily tuneable profile controlled by means of the probability density function associated to the period. We show that partially coherent plasmon modes have the remarkable property to control the length of propagation which is a notable feature respect to the completely coherent surface plasmon mode. The numerical simulation for sinusoidal, Bessel, Gaussian and Dark Hollow plasmon modes are presente
Parallel image scanning with binary phase grating
Get PDFWe discuss a novel approach for parallel image scanning, whereby the intensity peaks are generated by a binary phase grating instead of a lens array. These gratings under certain conditions generate one light spot per grating period in several transverse planes. The diameter of the focal spot is not restricted by the numeric aperture according to Abbe's law and can be used to scan a specimen with many spots simultaneously. Thus, imaging speed could be increased significantly. We discuss the principle effect and approaches to scanning as well as challenges for a potential implementation